1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus equipped with the imaging lens, and relates more particularly to an imaging lens that is appropriate, for example, for mounting in an onboard camera, a portable terminal camera and a surveillance camera which employ an imaging device, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), and an imaging apparatus equipped with this imaging lens.
2. Description of Related Art
Recently, imaging devices such as CCD and CMOS have been very much downsized and improved in pixel density. Subsequently, the availability and use of more compact imaging devices have made possible great reductions in sizes of image apparatus main bodies, and this, in turn, has created a continuing demand for smaller, lighter imaging lens for smaller, lighter imaging apparatuses.
On the other hand, with respect to onboard camera and surveillance camera applications, there is a demand for compact, high-performance cameras that are highly resistant to the adverse effects of wide ranging, extreme weather variations, from bitterly cold winter weather in the open air to overheated summer automotive interiors in tropical areas. Furthermore, from the viewpoint of automotive use, the onboard cameras most in demand are those for which exposed lens portions are the smallest.
Imaging lenses in the related art are described, for example, in JP-A-55-45007 and JP-A-61-90115. The imaging lens disclosed in JP-A-55-45007 is a wide-angle retro-focus lens including six lenses, which is used for photography, and the imaging lens disclosed in JP-A-61-90115 is an objective lens for image formation, including five or six lenses, for which a wide range of chromatic aberration corrections has been performed.
However, the F values of the lens systems described in JP-A-55-45007 and JP-A-61-90115 are 3.5 and 3.76 to 4.5, respectively, which means dark optical systems. Since it is anticipated that onboard and surveillance cameras will be used at night, in that case, a bright optical system is required.
Furthermore, according to the imaging lenses described in JP-A-55-45007 and JP-A-61-90115, the back focal length ratios relative to the thicknesses of the lens systems in the optical axial direction (the length in the optical axial direction from the lens surface nearest to an object to the lens surface nearest to an image) are large, i.e., about 0.95 and 1 to 2, respectively. That is, the entire length for the optical systems in the related art, from the lens system to the imaging surface is long, and the recent demand for the downsizing of lens systems is not satisfied.